Seedless watermelons have gained wide popularity due to their consumer appeal and high yield relative to seeded watermelon varieties. As a result, seedless watermelons may often garner above-average market prices. Responsive to heightened consumer demand and market pricing, farming operations have adapted to accommodate increased numbers of watermelon plants bearing seedless fruit.
The seedless trait of watermelons is a result of their triploidy, meaning the nucleus of each seedless watermelon cell contains three copies of each chromosome, instead of two. As such, seeds that produce seedless watermelons are herein referred to as “triploid seeds.” During meiosis, the odd number of replicated chromosomes in triploid cells often fail to properly segregate into daughter gamete cells, an outcome that prevents successful fertilization. Because seed formation requires successful fertilization, pollination of the gametes in triploid watermelon plants produces seedless fruit.
The modified genetics of seedless watermelons necessitate a specific pollination scheme. In particular, triploid seedless watermelon plants must be pollinated by the viable pollen produced by diploid watermelon plants bearing seeded watermelons. Because the production of seedless watermelons may be highly prioritized over seeded varieties, the required inclusion of seeded pollinizer plants in the field shrinks the area available for seedless watermelon plants and complicates planting systems. Growers typically strive to include the smallest proportion of seeded pollinizers relative to seedless varieties during sowing and transplanting operations to maximize the number of seedless watermelons grown in each field. On average, commonly-used diploid pollinizers may occupy approximately 20-33% of each field dedicated to growing seedless watermelons. This proportion of diploid pollinizers may generally provide adequate pollen for the seedless recipients grown concurrently in the same field.
Persistent germination difficulties have driven the sale of seedless watermelon varieties to commercial growers as young transplants instead of seeds. Generally, the seeds may be initially sown or planted in multi-cellular seedling trays and allowed to grow in commercial greenhouses. After a period of preliminary growth, the young watermelon plants are often transplanted into multiple fields. The survival of the young watermelon plants in the field may depend largely on their health and development prior to transplantation. Adequate root growth, in particular, may be the rate-limiting step to transplantation. Thus, the initial period of plant growth in seedling trays may be crucial to long-term plant survival.
Preexisting approaches to sowing watermelon seeds may involve sowing triploid watermelon seeds and diploid pollinizer seeds in separate seedling trays. The use of separate trays for pollinizer plants and seedless plants requires the trays be kept separate and well-marked during transport and during transplanting in the field. Other approaches may involve sowing both triploid seeds and pollinizer seeds within the same cells of a seedling tray, so-called “double plants.” Thus, despite efforts toward optimization, preexisting methods of sowing seedless watermelon plants remain tightly wed to particularly engineered seed varieties and often require specialized training of sowing personnel, thus rendering such methods inflexible and vulnerable to human error. Other methods limit the number of triploid seeds that can be grown in greenhouses and encumber the transition from greenhouses to fields. Improved sowing methods more universally applicable to a range of seed types, less vulnerable to human error, and yet capable of accommodating high volume production of seedless watermelons are needed.